The invention relates to catalysts for preparing cold- or hot-crosslinkable compositions.
Hot- and cold-crosslinkable compositions are prior art and their use as joint sealing compounds, adhesives, putties and filling compositions is known. Normally, compositions for said applications comprise a combination of functionalized binders, crosslinking agent and catalyst for accelerating the crosslinking reaction. In addition, modifiers may be added, such as fillers, plasticizers, dyes and the like.
Known, for example, are hydroxyl-terminated addition polymers which are crosslinked with diisocyanates as crosslinking agents and with a customary catalyst. Alternatively, a tetraalkyl silicate may be used as crosslinking agent, in which case tin(II) octoate is used as catalyst (U.S. Pat. No. 3,624,014).
EP 0 356 715 B1 relates to liquid or spreadable, vulcanizable rubber mixtures based on a plurality of polymers of which at least one is olefinically unsaturated, said mixtures comprising
from 30 to 70 parts by weight of a polymer A containing on average at least 2 hydroxyl groups per macromolecule
from 70 to 30 parts by weight of an A-compatible polymer B containing on average at least 2 carboxylic acid and/or carboxylic anhydride groups or at least one carboxylic acid and one carboxylic anhydride group per molecule
from 10 to 120 parts by weight of vulcanizing assistants and, if desired, fillers and/or auxiliaries, and
if desired, from 0.1 to 5 parts by weight of esterification catalysts, at least one of the polymers A or B being vulcanizable under customary reaction conditions at elevated temperatures by way of its olefinic double bonds, and only one of the components A or B deriving from a vulcanizable polydiene, the other component constituting, however, a polyamide which is compatible therewith, a polyester or an addition copolymer, in each case containing OH and/or carboxylic acid or carboxylic anhydride groups, or the polymers A and B deriving from polydienes or copolymers thereof.
EP 0 181 441 A1 describes a hot-crosslinkable composition and also a process for preparing a cold-crosslinking composition, consisting of a combination of two functionalized binders whose cold crosslinking is induced by addition of a catalyst. Binder I comprises an addition polymer based on a homobutadiene which has been appropriately functionalized with hydroxyl groups. Binder II is likewise an addition polymer based on a homo- or co-butadiene, which is provided, for example, with succinic anhydride groups. The polymers may be variously modified. For the actual crosslinking reaction, however, it is only the reaction of the hydroxyl groups with the acid anhydride group which is of importance. All other modifications serve in the widest sense to modify the product properties.
EP 0 181 441 A1 teaches that the cold crosslinking of such compositions may be catalyzed with preference by nitrogenous catalysts or sulfonic acids.
Nitrogenous catalysts described include, for example: pyridine, N-methylimidazole, 3-amino-2,4-dimethyl-pentane, di(2-ethylhexyl)amine. An example given of an appropriate sulfonic acid is p-toluenesulfonic acid.
The aforementioned compositions are employed, inter alia, in heat-curing adhesives for metal parts or for structural components which in the course of their processing are further subjected to heat treatment steps. The prior art catalysts have disadvantages for these applications. If metal parts are to be bonded by means of such compositions, the sulfonic acid, even in catalytic amounts, leads to an increased rate of corrosion at the areas wetted and thus to a reduced stability. Admittedly, this disadvantage can be circumvented by using amines. If, however, heat treatment is necessary in the subsequent course of processing, the amines too have disadvantages. At the amounts necessary for cold crosslinking (approximately 1% based on the overall amount of binder), elevated temperature is accompanied by evaporation of the volatile amine compounds and thus by a considerable odor nuisance in the course of processing.
It is an object of the present invention to provide a catalyst whose catalytic activity is similar to that of the known compounds but which is not corrosive toward metal parts and which does not lead to any odor nuisance in use.
The aforementioned object is achieved by means of catalysts for crosslinking cold- and/or hot-crosslinking compositions, comprising alkali metal salts of straight-chain or branched, saturated, mono- or polyunsaturated, aliphatic and/or aromatic carboxylic acids of natural and/or synthetic origin.
A starting point for achieving the proposed objective was the consideration that carboxylates might be suitable substances for catalyzing the reaction between an anhydride function and an alcohol function on the basis of their known catalytic activity in esterification reactions. In general, esterification reactions are conducted at temperatures above room temperature, which depending on ester type may range from approximately 80xc2x0 C. up to 280xc2x0 C. A catalysis at room temperature, on the other hand, is not known.
All alkali metal salts of aliphatic and aromatic carboxylic acids of synthetic or natural origin are suitable, including in each case alkyl- and/or aryl-substituted compounds and also mixtures thereof. Particular preference is given to those salts which are compatible with the polymer matrix and may be incorporated homogeneously, which is the case in particular for aliphatic carboxylic acid residues with a chain length from C3 to C22. Where necessary, the salts may also be used in the form of a solution in an inert solvent. Compounds of this kind are not corrosive toward customary metals and their alloys. Moreover, they are substantially odorless and have such a low volatility that even on exposure to heat they do not result in any odor-intensive evaporation from the polymer.
When test sheets of standard steel are coated with a composition comprising alkali metal carboxylate-crosslinked compositions and are stored for several hours at 80xc2x0 C. with exposure to atmospheric humidity, it is possible to ascertain neither any odor nuisance nor incipient corrosion of the metal sheets, as is observed, for example, under the catalytic effect of free acid.
The catalytic activity of the crosslinking reaction at room temperature between the hydroxy-functional polymer and the anhydride-functional polymer may be regulated by the concentration of alkali metal carboxylate that is used. For the target processing time of approximately 1 hour before the formation of a composition which is no longer flowable and is not tacky on the surface, a concentration of from 0.5 to 2.0 parts per 100 parts of polymer mixture, preferably from 0.7 to 1.5 parts, is sufficient. To achieve the same profile of properties, the amines or sulfonic acids known from the prior art are likewise used in a concentration of approximately 1 part per 100 parts of polymer mixture.
The suitability of alkali metal carboxylates for the use in accordance with the invention is surprising insofar as comparable carboxylates of polyvalent metal ions, such as those of zinc, of calcium or of bismuth, for example, do not exhibit sufficient catalytic activity at room temperature.